1. Technical Field
The present invention relates to a water purification process and more particularly, it relates to an improved process for the desalination of salt water by the strategic use of ion selective membranes to form a variable make-up that is fed to a desalination unit to produce a water product of potable quality.
2. Description of Related Art
The basic processes for the desalination of salt water to produce a water product of potable quality include, for example, multistage flash distillation, multieffect distillation, reverse osmosis and vapor compression distillation. Each of these processes are well established technologies having their own unique characteristics and limitations. The high content of scale forming ions, saline and other impurities found in untreated salt water has a negative impact on the efficiency, energy consumption and maintenance of large scale plants which employ any of these conventional desalination processes. Due to such high concentrations of impurities, it is commonly known to add antiscalling chemicals to the feed or equipment to minimize the adverse consequences of scale forming ions.
Multistage flash distillers which are in common use worldwide for large scale desalination projects are limited in their performance by the maximum brine temperature (top brine temperature or TBT) that can be used in the process. At temperatures higher than the maximum, scale inhibitors are ineffective and significant fouling of the internal surfaces will rapidly occur. This can be expensive and time consuming to remove.
Membrane technology has been used in the pre-treatment of salt water to reduce the high ionic content of salt water relative to fresh water. For example, U.S. Pat. No. 4,723,603 discloses a process for removing precursor ions from an injection water which formed insoluble salt precipitates in situ when they contacted resident ions already present in a subterranean hydrocarbon-bearing formation. The precursor ions of the insoluble salt precipitates are removed by means of a RO membrane.
More importantly for desalination, WO 99/16714 discloses the combination of membrane technology with the basic desalination processes to from a drinkable water product. According to this document, saline water containing a high content of hardness scaling ions is passed through a nanofiltration membrane to form make-up to a desalination system. Nanofiltration softening membranes are used for the selective removal of hardness scale forming ions and other impurities to soften seawater. Due to nanofiltration treatment, the make-up has a reduced ionic content when it passes through the desalination system. It is reported that there is a reduction of scaling and fouling tendency when this combination of nanofiltration and desalination systems is employed. However, the document does not suggest any means of taking advantage of the inter-relationship among certain conditions, e.g., pressure, temperature and make-up, to achieve an optimal system and recovery.
It will be seen that in spite of these disclosures, there is still a great need for a process that optimizes the combination of these two technologies to improves the operating conditions, efficiency and yield of this type of hybrid desalination systems.
Accordingly, the present invention is directed to an improved process for desalinating salt water at optimal operating conditions. In accordance with the invention, the content of hardness ions is sufficiently reduced from the make-up to a desalination system such that the desired top operating temperature, or desired recovery of potable water product, of any desalination system can be achieved. As a direct result, a number of advantages are realized which include a more cost-efficient operation of desalination plants, a reduction in the energy consumption of such plants and an increased yield of potable water. In addition, the use of chemical anti-scaling agents is advantageously minimized or completely eliminated. These and other advantages are achieved by specific improvements of hybrid desalination systems based on combined membrane and conventional desalination technologies.
As used throughout the following discussion, the term xe2x80x9csalt waterxe2x80x9d shall mean to include water having a significant concentration of hardness or scale forming ions, e.g., sulfate, calcium, magnesium and bicarbonate ions. Sources of salt water which are contemplated by the present invention include, without limitation, ocean water, gulf water, reject, blowdown and recycle brine in solution and impaired water containing soluble salts having an ionic content of hardness ions in excess of 1,500 mg/liter.
In accordance with the present invention, a first stream of salt water containing a high concentration of hardness ions is passed through an ion selective membrane to form a softened salt water product having a reduced content of hardness ions. The softened salt water is blended with a second stream of salt water containing a high concentration of hardness ions to form a feed to a desalination system. The feed is then introduced to the desalination system to form a water product of potable quality. The softened salt water content of the feed is at least 5%. The desalination system may be one or more desalination processes including reverse osmosis, multistage flash distillation, multieffect distillation and vapor compression distillation.
In one embodiment of the invention, a nanofiltration (NF) membrane is employed as the ion selective membrane. A NF membrane softening system comprising one or more of such NF membranes is combined with a multistage flash (MSF) distillation plant. The NF system is introduced after the MSF-dearerator. In one embodiment, untreated saltwater is first subjected to a deaeration pre-treatment step before entering the NF system. Alternatively, a stream of softened sea water exits the NF system and then enters the MSF-dearerator for the removal of non-condensable gases from the softened water. A portion of the stream of untreated salt water is pre-heated by the heat of a reject stream exiting the reject section of the desalination plant before passing through the NF membrane system. The MSF distillation system is operated at a TBT of 95-180xc2x0 C. The NF membrane system is operated at a variable pressure of 5-60 bar. To be discussed in more detail, the ability to vary the operating pressure of the NF system provides a means of control over the ionic content and quantity of softened salt water.
In another embodiment of the invention, the softened salt water is stored in a buffer system from where it is blended with a stream of untreated salt water to form the feed to the desalination system. Alternatively, the softened salt water that is stored in the buffer system is injected into the desalination system. Additionally, this reserved supply of softened salt water is used to form a cluster feeding system wherein the softened salt water is fed from the buffer system to two or more desalination systems for blending with untreated salt water.
In yet another embodiment, a partial stream of reject brine, blowdown or recycled brine produced during the desalination process is subject to a nanofiltration step and recycled through the desalination system.